Compensation for Goos-Hanchen error in autofocus systems

ABSTRACT

Prediction of a distribution of light in an illumination pupil of an illumination system includes identifying component(s) of the illumination system the adjustment of which affects this distribution and simulating the distribution based on a point spread function defined in part by the identified components. The point spread function has functional relationship with configurable setting of the illumination settings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation from U.S. patent application Ser. No.14/302,187, now published as U.S. 2014/0293278, which is a continuationfrom U.S. patent application Ser. No. 12/884,890, now abandoned, whichin turn claims the benefit of and priority from the U.S. ProvisionalPatent Application No. 61/244,321, filed Sep. 21, 2009, whichprovisional application is incorporated by reference herein.

BACKGROUND

The present invention provides a method for compensating errors due tothe Goos-Hanchen effect in an autofocus (AF) system.

The Goos-Hanchen (GH) effect produces a shift of a beam when incident onan optical interface (e.g. a substrate that is imaged by an imagingoptical system in the production of a semiconductor wafer). In one wayof looking at this effect, any monochromatic beam incident on areflecting surface can be decomposed into a sum of plane waves. Thereflecting surface (e.g. the substrate surface) then produces adifferent phase for each plane wave depending on its angle of incidence.Very often, over a small range of angles, this phase on reflection willeither increase or decrease with the angle of incidence producing atilted wavefront in the far field, which is the same as a shifted spotat the reflecting surface—the near field. In an imaging optical systemthat includes a reflective surface near an image, this effect willproduce a shift of the image. This is also true in an autofocus systemthat images some source object (e.g. a slit or fringes) onto the surfaceof investigation (e.g. a wafer) at a glancing angle of incidence andthen relays that image to a detector. The position of the image on thedetector will depend on the height of the surface of investigation, butwill also depend on the variation of phase on reflection produced bythat surface—the GH effect. In an AF system, this means that variationsin the surface construction, which may consist of many thin film layersand printed circuit patterns, will produce an error in the surfaceheight measurement; we call this the GH error.

The problem with the GH error is that it can vary with underlyingsubstrate patterns, and coating thicknesses, and that variation can belarge, e.g. several hundred nanometers to several microns. Moreover,that variation is typically indistinguishable from the substrate(substrate) topography in an optically based AF system.

One approach to compensating the GH-effect is to use ellipsometry todetermine the substrate film structure, and then use the film structureto estimate the GH error, and finally subtract that error from themeasured surface height. However, ellipsometry requires a complexoptical system of its own, a big increase in computational power, and alot of input from the user.

SUMMARY

The present invention provides a method for compensating errors causedby the Goos-Hanchen effect in an optical autofocus system that uses theposition of an image reflected from a substrate (e.g. wafer surface) todetermine changes in the z position of the substrate. According to theinvention, reflected light from the substrate is provided at a pluralityof wavelengths and polarizations, detected and used to make correctionsthat compensate for the errors due to the Goos-Hanchen effect.

One way of compensating GH errors, according to the principles of thepresent invention, is referred to as the “analog” approach. In thisapproach, a broad band light spectrum is directed at the substrate, sothat the variation of GH error is minimized across various substratepatterns. This approach is already deployed, in a non-optimized way, invery broadband AF systems, where the influence of spectrally isolated GHerrors are reduced by the more ubiquitous spectral components that havesmaller GH errors. However, according to the present invention, thebroad band illuminating spectrum is filtered, e.g. with a dynamicfilter, or with a custom interference filter before reaching thedetector so that the average GH error (averaged across wavelength andpolarization by the detector) is minimized. This key idea behind thisapproach is that the GH error is an average over the spectrum andpolarizations and that no further specialized data processing isnecessarily used in correcting the GH error. We therefore call it the“analog approach”.

Another embodiment comprises a slight modification to the analogapproach. In this case the position of the imaged source object, eachwavelength, or wavelength band, and/or polarization receives a shiftthat biases the measured substrate position for that wavelength,wavelength band and/or polarization so that the average position isfurther compensated for GH errors. Such a bias could be achieved in afringe projection system using a modified spectral filter that, inaddition to attenuating the light as a function of wavelength, alsoapplies a dynamic and differential phase shift (between +1 and −1orders), which will shift the image of the source object on the detectorfor the wavelength, wavelength band and/or polarization concerned.

Another way of compensating GH errors, according to the principles ofthe present invention, is referred to as the “digital” or “digitalfilter” approach. With this approach, rather than modifying the spectrumin the incident and reflected light, a spectral and polarization filteris applied in software after each wavelength, wavelength band and/orpolarization is detected separately in space, time or angle inaccordance with the principles of the present invention. A singlebroadband spectrum, or many narrow band spectra, or a combination ofbroad and narrow band spectra are used to illuminate the substrate. Thecombined spectrum is then separated into several sub-bands andpolarizations that are directed at one or more detectors that sense theposition of the substrate and possibly its reflectance as a function ofwavelength, polarization. Then the AF position is estimated with aweighted average among the spectral and polarization components, wherethe weighting (known as a digital filter) is made to reduce the overallvariation in GH error across various substrate conditions for a givenprocess (and/or a given imaging optical system). This type of system andmethod can be used with a fixed optical system with few or no movingparts.

In one version of the digital approach, broadband unpolarizedillumination is used in imaging the object (e.g. slits or fringes) tothe substrate. The light leaving the substrate is then separated intodifferent polarizations and wavelengths that are then detectedseparately. The key point here is that the separation is done afterreflection from the substrate. In this version of the method, acombination of dichroic and polarization beam splitters can be used toseparate the measurements. In another embodiment, gratings can be usedto perform the chromatic separation. In another preferred embodiment,the chromatic separation can be performed by a pair of prisms, a firstprism that spreads the reflected light as collimated light in angles bywavelengths, and a second prism that is displaced from the first prismalong the z axis, and makes the collimated rays at all the wavelengthsparallel, so that the wavelengths are spatially separated, but theirdirections maintained. In either case of gratings, or prisms used toperform the chromatic separation, polarization beam splitters can beused to perform the polarization separation. In another embodiment, thepolarization separation can be performed with polarizing elements placeddirectly in front of the detector elements that receive duplicate imagesof the of the source object.

In another version of the digital approach, the source object isilluminated by light that contains a plurality of wavelength bands suchthat each wavelength band is well separated in the far field image ofthe object. In this case, the different wavelengths can be picked off inthe pupil of the relay optics following reflection by the substrate.With the wavelength well separated in the pupil, a set of mirrors orprisms can be used to direct each wavelength band to differentdetectors. In a preferred embodiment, a set of tilted mirrors is used totranslate the image to different areas of a CCD. In this approach,polarization can be separated in the same way, or as is preferred, by apolarizing beams splitter that sends the beam to two separate CCDs.

In yet another version of the digital approach, the source object isilluminated sequentially in time by a plurality wavelength bands andpolarizations. In this embodiment, the measurements at each polarizationand wavelength band are also made sequentially in time.

In all of the disclosed versions of the invention, it is preferred thatsource object, that being imaged onto the substrate, and relayed to adetector, comprise a set of sinusoidal fringes produced by two-beaminterference. Such fringes can be generated by illuminating a lineargrating having twice the desired periodicity, and filtering the farfield image such that only the +1 and −1 orders are allowed to reach thedetector.

The present invention takes advantage of the fact that the GH error issignificantly different across wavelengths and polarizations. Because ofthis, different spectra have different amounts of GH error for differentsubstrate structures. And measurements made at a plurality ofwavelengths and polarizations similarly contain information about thosesubstrate structures.

Thus, the present invention compensates GH errors with or withoutdetailed information about the substrate, without the complexities ofellipsometry (it can be thought of as an approximation or short-cut toellipsometry), in a manner such that the GH error can be reduced toalmost arbitrarily low levels.

Further aspects of the present invention will become apparent from thefollowing detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a fringe type projection system,with which the present invention is particularly useful;

FIGS. 2a and 2b are schematic illustrations of one way of compensatingthe GH effect, in accordance with the principles of the presentinvention;

FIGS. 3a and 3b are schematic illustrations of additional aspects ofcompensating GH errors in an autofocus system, in accordance with theprinciples of the present invention; and

FIG. 4 is an illustration of a pair of prisms that can be used on thedetection side of a system in practicing the principles of the presentinvention.

DETAILED DESCRIPTION

As described above, the present invention provides a new and usefulconcept for compensating errors due to the Goos-Hanchen (GH) effect inan autofocus (AF) system. The principles of the present invention areparticularly useful in compensating GH errors in a fringe type AFsystem, and are described herein in connection with such a system.However, from that description, the manner in which the principles ofthe present invention can be applied to other AF systems (e.g. systemsthat use slit type detection) will be apparent to those in the art.

FIG. 1 schematically illustrates the principles of an autofocus (AF)system and method, using fringe projection. The system has a sendingside 100, from which light is directed at a substrate 102, and areceiving side 104, wherein light reflected from the substrate isdirected to a detector 106. On the sending side 100, the light (e.g.broadband or “white’ light) is reflected from a fringe generator 108,filtered by a filter 110 and reflected from the substrate 102. Thefringe(s) of the reflected light is (are) detected at the detector 106,and used to determine the initial position of the substrate surface inthe z direction. Subsequent operation of the AF system is then used todetermine changes in the z position, and those changes may be used tocontrol movement of the stage that supports the substrate in the zdirection. The “fringe shift”, i.e. the amount by which the fringe isshifted, may be defined by y=2 m²z Tan θ, where z is the z position ofthe substrate, θ is the angle of incidence (shown in FIG. 1), m is themagnification between the substrate and detector, and y is the fringeposition along the detector (shown in FIG. 1).

There are two (2) basic approaches to the method by which Goos-Hanchenerrors may be compensated, according to the principles of the presentinvention; one is referred to herein as the “analog” approach, and theother is referred to herein as the “digital” or “digital filter”approach.

The analog approach is an extension of the idea that a different spectraproduce different amounts of Goos-Hanchen (GH) error for a givensurface, and that there is some spectrum that will minimize this error.Therefore to compensate the GH error, the method of the inventionprovides for adjusting the input spectrum, either by filtering thespectrum (and polarization) of a broad band source, or by varying theamount of light that is allowed to pass from a set of relatively narrowband sources. A related approach, which can still be referred to as ananalog approach, is to introduce a wavelength (and polarization)dependent shift to fringes in fringe projection systems (or to the slitimages in slit projection systems). This allows the implementation ofnegative spectral components. In practice a combination of theseapproaches may be beneficial. In these versions of the analog approach,the average autofocus signal (averaged over all wavelengths) willcontain reduced GH error, and has the convenience of using few detectorelements relative to the digital method.

In the digital approach, the optical AF signal is divided into spectral(and polarization) components at the detector. This can be done in timeby pulsing the sources and alternating/rotating the polarization state(this could also be done to the light after it is incident on thesubstrate with appropriate chopping and/or switching mechanisms), or byseparating the image spatially and sending the different wavelengths(and polarizations) to different detector elements—producing a pluralityof AF measurements for a single position on the substrate. Once theplurality optical AF measurements have been made for a single position,they are combined by a weighted sum, where the weightings have beenchosen to reduce the dependence on the GH error, much in the same waythe spectrum was chosen in the analog approach.

In choosing the weights or the spectra; in order to effectively reducethe GH error, a set of weights or spectra shapes that achieve this goalmust be found, and there are several possible approaches. For example,one way to determine the GH error as a function of the spectra orweights is by simulation, and then determining the best spectra or setof weights by some sort of optimization—e.g. simulated annealing, ordamped least squares. Another approach, is to make a set of chromaticand polarization separated optical AF measurements on the targetsurface, and then also measure the surface by some other method thatdoes not have GH errors (like an air-gauge, or touch profilometer) andthen find the set of weights that reduces the GH error. This approach iseasily amenable to a least squares solution and one skilled in the artwill easily see that.

Another way to characterize the digital approach is that each wavelength(and polarization) is used to measure the z position of the substrate.This gives a set of measured z positions. Then, the weighted average(set of a and b coefficients) are applied to the set of z positions togive a single, final, corrected measurement of the substrate z position.In order to know the coefficients a, b, the AF system must be calibratedfor a given substrate type. This can be done in at least 2 ways:

-   a) Use a physical sensor (air gauge, etc) to measure the height at    some set of positions on the substrate and compare this with the    results from the optical autofocus system. The air gauge or other    physical sensor can't be used for every substrate because it is much    slower and has a hard time measuring many points. This is done once    per process, establishing the set of {a, b}-   b) Print a test substrate for a given process and use the result of    the printing (which used the data from the optical AF to establish    the substrate at the right z height during printing) to determine    the error of the optical AF, and use this to determine the set of    {a, b}.

In the digital approach, it is possible to use other information (suchas the amount of reflected light, for example) to sub-classify differentregions on the substrate. Say half the substrate has copper (process A)and the other half has no copper (process B). There could be 2 sets ofcoefficients {a, b}, that could be preferentially applied depending onwhich region is being measured.

Also, in the digital approach, the light reflected by the substrate isused to determine changes in the z position of the substrate by applyinga weighting average to the set of z measurements at the differentwavelengths (λ's) and polarizations (e.g. s and p polarizations), andusing the weighting average to make corrections that account for theGoos-Hanchen effect. Moreover, the weighting average is produced by alinear least squares regression estimate of the coefficients of thefirst order position of the substrate, preferably according to thefollowing formula

$Z_{j} = {a_{0} + {\sum\limits_{n = 1}^{N}{\sum\limits_{{v = s},p}{a_{k,v}z_{j,n,v}}}} + {\sum\limits_{m = 1}^{M}{\sum\limits_{{v = s},p}{b_{k,v}r_{j,m,v}}}}}$where

-   Z_(j) is the substrate height at position j on the substrate-   a_(o) is a DC offset value-   a_(k) is the set of coefficients for the z_(j)(lambda) measurements,    where there is a different j for each wavelength

$\sum\limits_{{v = s},p}$

This is a sum over the s and p polarization measurements

-   M is the number of wavelength bands-   r_(j,m,v) is the reflectance at    -   j^(th) position    -   m^(th) wavelength band (or sub-spectrum)    -   v^(th) polarization-   z_(j,n,v) is the z height at    -   j^(th) position    -   n^(th) wavelength band    -   v^(th) polarization-   b_(k) is the set of coefficients for the reflectance measurements of    the substrate, r_(j)(lambda). The same data collected and used to    measure z will also be used to calculate wavelength dependent values    for r, the reflectance. The set of b's are the weighting    coefficients for making a correction using this data.-   For a and b, the v subscript is for the two polarization states (s    and p).

By comparison, the analog approach would use the same type ofcalibration for determining the set of {a, b}, but it would be reallyhard to have spatially varying sets of coefficients (say, 2 sets forprocess A and B). The implementation of the filter would be done byattenuating the light at the various wavelengths and polarizations usinga mechanical shutter, or filter. Then, the detector would measure z onlyonce, and that measured z would be the result of applying the {a,b}coefficients directly on the light, rather than on the digital,calculated versions of z. The potential advantage of the analog methodis that the data collected could be much less and the corrected zmeasurement could be determined very quickly.

FIGS. 2a and 2b schematically illustrate one version of providing an AFsystem and method that compensates errors due to the GH effect, which isparticularly useful with the digital approach described herein. In thesystem and method of FIGS. 2a and 2b , the light is directed at thesubstrate 102 at a plurality of wavelengths (λ's), the light at theplurality of wavelengths is reflected from the substrate (and from areference mirror) and broken into different polarizations, and thendetected. In this version of the method, separate light sources 107 offinite spectral width are incident on the substrate 102 (and on areference mirror 115) at different angles such that they can beseparated in angle space. The light from the sources is reflected from afringe generating grating forming part of a fringe generating module 108(and also from a reference region of the fringe generating module), anddirected through a filter 110 before it is reflected from the substrate102 and the reference mirror 115. The reflected light at the differentwavelengths is spatially separated by prisms 109, and then separated bypolarization by means of a polarization beam splitter PBS 105. Thus,reflected light at the different wavelengths and polarizations isdirected to respective detectors 106 a, 106 b.

In another version of an autofocus system and method, according to theprinciples of the present invention, which is also particularly usefulwith the digital approach described herein, and shown schematically inFIGS. 3a, 3b , and 4, broadband light (or “white”) light from abroadband source is directed at a substrate 102 (and also at a referencemirror 115), (through a fringe generating module 108 and a filter 110,that are similar to the prior embodiment and therefore not shown).Broadband light reflected light from the substrate 102 and the referencemirror 115 is then magnified (e.g. by a magnification relay 111), brokeninto different polarizations and wavelengths that are then detected. Inthis version of the method, it is preferred that the broadband lightreflected from the substrate (and the reference mirror) is refracted bya pair of prisms 117 a, 117 b, a first prism 117 a that spreads thereflected light as collimated light in angles by wavelengths, and asecond prism 117 b that is displaced from the first prism along the zaxis, and makes the collimated rays at all the wavelengths parallel, sothat the wavelengths are spatially separated. The light is furtherseparated by polarization, by polarization beam splitters similar to 105(FIG. 2a ), and detected by detectors 106 a, 106 b.

In yet another version of an autofocus system and method, according tothe principles of the present invention, which is also particularlyuseful with the digital approach described herein, the system would beset up in a manner similar to that shown in FIGS. 2a, 2b , but lightthat is directed at the substrate 102 comprises light from anillumination source that produces light sequentially at a plurality ofwavelengths and a plurality of polarizations, and light at the pluralitywavelengths and polarizations is detected sequentially. In this version,there would not be a need for the plurality of prisms 109 shown in FIGS.2a , 2 b.

In all of the disclosed versions of the invention, it is preferred thatsource object, that being imaged onto the substrate, and relayed to adetector, comprise a set of sinusoidal fringes produced by two-beaminterference. Such fringes can be generated by illuminating a lineargrating having twice the desired periodicity, and filtering the farfield image such that only the +1 and −1 orders are allowed to reach thedetector.

Thus, the basic concept of the present invention corrects for GH errorin an AF system and method, by detecting light reflected from thesubstrate at different wavelengths and different polarizations, andusing the detected light to compensate GH error. The principles of thepresent invention can be applied to either slit detection or fringedetection, but fringe detection is currently preferred. However, fromthis description, the manner in which the principles of the inventioncan be practiced with slit detection will be apparent to those in theart.

Also, in applying the principles of the present invention to an AFsystem and method, it should be noted that light directed at thesubstrate is preferably in a spectral range of 400 nm to 1000 nm so thatit can work with the most commonly available glasses and detectors, butin principle can be any range of wavelengths that do not damage or alterthe surface under investigation. The manner in which the light isdirected at the substrate at a plurality of wavelengths andpolarizations can take a number of forms, such as a single broadbandlight source that is filtered to produce the different wavelengths anddifferent polarizations, or a plurality of light sources that producelight in narrow bands, and at different polarizations. The light at theplurality of wavelengths and polarizations is directed at the substrate(and the reference mirror), and reflected light from the substrate (andthe reference mirror), at the plurality of wavelengths andpolarizations, is directed to one or more detectors. The detected lightis used to determine changes in the z position of the substrate beingimaged. Preferably, the invention contemplates separate detection foreach wavelength and each polarization. Moreover, the detector(s) canalso measure the reflectance of the substrate, which is useful with thedigital filter aspect of this invention, described further below.

The “analog” approach uses a broadband light source, and filters (e.g.dynamic filters or custom filters), or a number of multiplexed broadbandor narrowband sources, to modify the spectrum (both wavelength andpolarization) so that the variation of GH error is minimized acrossvarious substrate patterns. Thus, a broad band illuminating spectrum isfiltered with a dynamic filter, or with a custom interference filter, toproduce light at different wavelengths and polarizations in the lightdirected at the substrate and/or the light reflected from the substrate,and each of those different wavelengths and polarizations is detectedsimultaneously. Each sub-band and polarization is biased by themeasurement system, so that, for example, individual wavelength bandsand polarizations contain more or less power relative to the others andmay also overestimate or underestimate the substrate position so thatthe final measurement has reduced sensitivity to GH shifts.

With the “digital filter” approach, a spectrum filter is applied insoftware. On the “sending” side of the substrate (i.e. from which lightis directed at the substrate), the light is produced by a broadbandsource (that is filtered, e.g. by a turret filter) or several discretelight band sources, to direct light at the substrate at differentwavelengths and at the different polarization (i.e. s and ppolarizations) and possibly different angles of incidence. On the“receiving” side of the substrate (i.e. which receives the reflectedlight from the substrate) reflected light at the different wavelengthsand polarizations is directed at one or more detectors, and the light atthe different wavelengths and polarizations is separately measured atthe detectors. The information from the detectors can then be used todetermine variations of the z position of the substrate. The detector(s)can take various forms, e.g. CCD (charged couple device), individualslit detectors, etc., which would collect data so that the phase of theprojected fringes can be calculated and the substrate height can bemeasured. Prisms or gratings are used to separate the fringes intofinite wavelength bands, e.g. corresponding to pixel columns on the CCD.This allows for the use of a digital filter approach, as describedherein.

The digital filter approach can be practiced with a serial or parallelapproach, in terms of the way light is handled at the sending andreceiving sides of the substrate. The “serial” approach provides asingle detector that is sensitive to multiple wavelengths andpolarizations on the receiving side, and a sending side that is switchedbetween wavelengths and polarizations sequentially in time. The serialapproach allows for switching between sources at discrete bands, orproviding filters with broadband sources (e.g. turret type filters), orby switching between different sources having different spectra. Iffilters are used they can be on the sending and/or the receiving side ofthe substrate. With the “parallel approach”, all wavelengths andpolarizations are projected simultaneously at the substrate; thewavelengths and/or polarizations may be split at the receiving side ofthe substrate and directed to multiple detectors. Then the AF positionis estimated with a weighted average among the spectral and polarizationcomponents, where the weighting (analogous to a digital filter) is madeto reduce the overall variation in GH error across various substrateconditions for a given process.

As specifically illustrated in FIGS. 3a, 3b and 4, where a broadbandsource is reflected from the substrate, in order to separate thewavelengths, a pair of prisms is used (117 a, 117 b). The first prism(117 a) spreads the collimated light out in angle by wavelength. Thesecond prism (117 b) makes rays from all wavelengths parallel again, butsince the second prism is displaced along the z-axis from the firstprism, the colors have been spatially separated. This light is thenincident on the detector(s).

With a system and method using the principles of the present invention,because the GH error is strongly dependent on wavelength, a small changein the spectrum of a broad-band source can have a noticeable effect onthe error. Thus, the principles of the present invention contemplateusing a spectrum where the GH error is minimized, modifying theilluminating spectrum with some sort of dynamic filter, measuring theslits or fringes at several wavelength bands and apply the spectrum insoftware as a digital filter.

The digital filter approach is particularly attractive (and is thereforecurrently preferred) because phase and reflectivity information can begenerated, as well as irradiance (light intensity), all of which arerelated to changes in the z-position of the substrate and can easily beincorporated into a digital filter.

A system and method that practices the principles of the presentinvention can be thought of as a short-cut to ellipsometry where,instead of a priori knowledge, measurements from a sensor that isrelatively immune to the GH effect can be used to determine the filterthat best matches the measured z-position. Moreover, it may be possibleto use the optical sensor data from several substrates to find a filterthat minimizes the variation, possibly allowing for low orderdeformations of the substrate surface. In either case, the method couldbe largely automated.

With the digital approach, the method of finding the spectral filter iscurrently a linear least squares estimate of the coefficients of thefirst order position of the substrate as function of wavelength band andpolarization, but it is recognized that there will be other approachesthat can be used, and that higher order regression models involvingother measurable quantities like reflectance can be included in thefilter.

In one preferred embodiment of the digital filter approach, the optimalfilter is calculated from a set of measured substrate heightsz_(j)(□_(kv)) where j (from 1 to N) indicates the location of themeasurement on the substrate, □_(kv) indicates the wavelength band k(from 1 to M) and polarization state v (e.g., either s, or p). At eachof these locations, there is also a known substrate height Z_(j), whichmay be measured by an independent system that is relatively immune tothe GH effects. Given this notation, we desire an estimate of the knownheight Z_(j) in terms of the spectrally measured heights z_(j)(□_(kv)).The simplest solution form is a linear combination of the measuredheights,

$Z_{j} = {a_{0} + {\sum\limits_{k = 1}^{M}{\sum\limits_{{v = s},p}{a_{kv}{{z_{j}\left( \lambda_{kv} \right)}.}}}}}$

One approach for determining the coefficients is linear least squaresregression using the following regression model (which is furtherdescribed above).

$Z_{j} = {a_{0} + {\sum\limits_{n = 1}^{N}{\sum\limits_{{v = s},p}{a_{k,v}z_{j,n,v}}}} + {\sum\limits_{m = 1}^{M}{\sum\limits_{{v = s},p}{b_{k,v}{r_{j,m,v}.}}}}}$where

-   Z_(j) is the substrate height at position j on the substrate-   a_(o) is a DC offset value-   a_(k) is the set of coefficients for the z_(j)(lambda) measurements,    where there is a different j for each wavelength

$\sum\limits_{{v = s},p}$

This is a sum over the s and p polarization measurements

-   M is the number of wavelength bands-   r_(j,m,v) is the reflectance at    -   j^(th) position    -   m^(th) wavelength band (or sub-spectrum)    -   v^(th) polarization-   z_(j,n,v) is the z height at    -   j^(th) position    -   n^(th) wavelength band    -   v^(th) polarization-   b_(k) is the set of coefficients for the reflectance measurements of    the substrate, r_(j)(lambda). The same data collected and used to    measure z will also be used to calculate wavelength dependent values    for r, the reflectance. The set of b′s are the weighting    coefficients for making a correction using this data.-   For a and b, the v subscript is for the two polarization states (s    and p).

In an extension of this approach, nonlinear combinations of the variousmeasurements may be included in the model. For example, the square ofeach z-measurement z_(j)(□_(kv)), or cross terms like z_(j)(□_(kv))z_(j)(□_(h)). Similarly, cross terms between reflectance measurementsand cross terms between z and r can be used to make the correction, toany order deemed useful by the usual techniques of regression analysisor by careful analysis of the ellipsometric relationships.

A further extension of this approach could include measurements ofsubstrate height and reflectance at multiple angles of incidence and/orsubstrate orientation (i.e. clocking).

Also, one could use a nonlinear regression, for example with polynomialbasis as follows:

$Z_{j} = {a_{0} + {\sum\limits_{n = 1}^{N}{\sum\limits_{q = 1}^{Q}{\sum\limits_{{v = s},p}{a_{k,v}z_{j,n,v}^{q}}}}} + {\sum\limits_{m = 1}^{M}{\sum\limits_{v = 1}^{T}{\sum\limits_{{v = s},p}^{\;}{b_{k,v}r_{j,m,v}^{t}}}}}}$

It is also preferred that the projected pattern (either slits orfringes) at all wavelengths have the same period and phase, and that allof the wavelengths are looked at simultaneously, or separated with aprism/grating such that each band corresponds to one region of an areadetector (e.g., a charge coupled device (CCD)). Those principles wouldutilize, e.g., a mirror array placed conjugate to the substrate.

Thus, the foregoing description provides several new and useful ways ofcompensating for errors due to the Goos-Hanchen effect in an opticalautofocus system and method that uses light reflected from a substrateto determine changes in the z position of the substrate. The correctionis performed through the use of reflected light from the substrate at aplurality of wavelengths and polarizations that is detected and used tomake corrections to the z position of the substrate that compensate forthe errors due to the Goos-Hanchen effect. From the foregoingdescription, other ways of compensating for errors due to theGoos-Hanchen effect in an autofocus system and method, using theprinciples of the present invention, will become apparent to those inthe art.

The invention claimed is:
 1. A surface position detecting apparatus, comprising: a projection system configured to project light onto a surface of a chosen workpiece to form a pattern of light distribution thereon the pattern including at least one first area and at least one second area on the surface, the first area having a first level of illumination and the second area having a second level of illumination, the first level being lower than the second level; a light detecting system including: a polarization filter unit configured to receive said light in reflection from the surface of the workpiece and to separate said light into a first light portion having a first state of polarization and a second light portion having a second state of polarization, the first and second states of polarization being different from one another, a first optical detector unit disposed (i) to acquire the first light portion, (ii) to detect, in said first light portion, a first distribution of light corresponding to said pattern, and (iii) to generate a first output signal in response to having detected said first distribution of light; a second optical detector unit disposed (a) to acquire the second light portion (b) to detect, in said second light portion, a second distribution of light corresponding to said pattern, and (c) to generate a second output signal in response to having detected said second distribution of light; wherein the light detecting system is situated to receive the light reflected by said surface after obliquely impinging onto the surface from an oblique direction, and a controller unit operably cooperated with the light detecting system and configured to calculate a position of the surface of the workpiece based on the first and second output signals.
 2. The surface position detecting apparatus according to claim 1, configured to cause said first light portion to undergo a first Goos-Hanchen shift resulting from interaction of said light with the workpiece, and to cause said second light portion to undergo a second Goos-Hanchen shift resulting from interaction of said light with the workpiece, wherein the first and second Goos-Hanchen shifts are different from one another.
 3. The surface position detecting apparatus according to claim 1, wherein the controller unit is configured to reduce an error, contributed to said position of the surface of the workpiece as a result of a Goos-Hanchen shift experienced at the workpiece by said light, with the use of the first and second output signals.
 4. The surface position detecting apparatus according to claim 1, wherein the light detecting system includes: a first condensing optical system configured to spatially condense light received from the surface of the workpiece; a second condensing optical system configured between the polarization filter unit and the first optical detector unit to: (i) cause the surface of the workpiece and a detecting surface of the first optical detector unit to be optically-conjugate to one another; and (ii) spatially condense the first light portion; and a third condensing optical system configured between the polarization filter unit and the second optical detector unit to: (a) cause the surface of the workpiece and a detecting surface of the second optical detector unit to be optically-conjugate to one another and (b) to spatially condense the second light portion.
 5. The surface position detecting apparatus according to claim 4, wherein the projection system includes a light source configured to emit said light, said light being polychromatic.
 6. The surface position detecting apparatus according to claim 1, wherein the light detecting system includes an optical system arranged across said light between the first condensing optical system and the polarization filter unit and configured to spatially split said light received from the surface of the workpiece according to wavelengths to form a plurality of spatially-different beams having different wavelength content and directed towards the polarization filter unit.
 7. The surface position detecting apparatus according to claim 1, further including a light source configured to emit said light, and wherein the projection system includes a diffraction grating disposed to receive said light and diffract it to form diffracted light; and a light sending optical system between the diffraction grating to condenses the diffracted light on the surface of the workpiece to form said pattern thereon.
 8. The surface position detecting apparatus according to claim 7, wherein the projection system further include a spatial filter disposed to transmit positive and negative diffraction orders of said diffracted light and to block the 0th diffraction order thereof.
 9. The surface position detecting apparatus according to claim 8, wherein the spatial filter is configured to transmit a +1 diffraction order and a −1 diffraction order of said diffracted light.
 10. The surface position detecting apparatus according to claim 8, wherein the light source is configured to emit said light at a plurality of wavelengths, and wherein the positive diffraction orders and the negative diffraction orders for said plurality of wavelengths pass through the spatial filter.
 11. The surface position detecting apparatus according to claim 1, wherein the pattern is projected to a measurement area on the surface of the workpiece.
 12. The surface position detecting apparatus according to claim 11, wherein the pattern includes a spatially-periodic pattern having a period along a longitudinal direction of the measurement area.
 13. An exposure apparatus configured to expose said workpiece to light, the apparatus comprising: the surface position detecting apparatus according to claim 1; and a stage configured to hold said workpiece.
 14. The exposure apparatus of claim 13, wherein the stage is configured to change at least one of orientation and position of the workpiece, the exposure apparatus further comprising a controller configured to control the at least one of the orientation and position of the workpiece based on an output from the surface position detecting apparatus.
 15. A manufacturing method comprising: exposing the workpiece to light by using the exposure apparatus of claim 13 to form an exposed workpiece carrying a predetermined pattern on the surface of the workpiece; developing the exposed workpiece; forming a mask layer dimensioned to have a form corresponding to the predetermined pattern on the surface of the workpiece; and processing the surface of the workpiece via the mask layer.
 16. The surface position detecting apparatus according to claim 1, wherein each of the first and second light portions contains a plurality of beams that have been spatially-separated from one another according to wavelengths of said light at a location between the surface and the polarization filter unit.
 17. The surface position detecting apparatus according to claim 1, wherein the projection system includes an optical objective positioned across a first beam of said light and configured to transform said first beam, upon transmission therethrough, into a first illuminating beam and a second illuminating beam that form the at least one first area and the at least one second area on the surface of the chosen workpiece.
 18. The surface position detecting apparatus according to claim 1, wherein a detecting surface of at least one of the first and second optical detector units is tilted with respect to an optical axis of a beam of light incident thereon from the polarization filter unit.
 19. The surface position detecting apparatus of claim 1, wherein the at least one first area includes multiple first areas and the at least one second area includes multiple second areas, wherein the multiple first areas and the multiple second areas alternate across the surface, the multiple first areas having the first level of illumination and the multiple second areas having the second level of illumination. 